It is well known that sewage sludge can be solidified and rendered biologically suitable for use as landfill by mixing certain chemicals therewith, principally lime or fly ash or both. One manner of accomplishing this is to haul the sludge to the site where it is to be used as fill, spread it on the land, and then till or plow the chemicals into it. This enables relatively little control as regards the uniformity of the mix attained. It is also known to pass the sludge and chemicals through a covered stationary receptacle having motor-driven mixing paddles therein which mix the chemicals with the sludge. One of the problems with such a system is that the mixing paddles which move with respect to the receptacle, and the necessary power shaft inputs and associated seals, are subject to considerable wear and tear. Further, since the reaction of the chemicals with the sludge can be exothermic and generate vapors such as steam which can escape into the atmosphere, there can be a problem of polluting the atmosphere with noxious ingredients. U.S. Pat. No. 4,079,003 shows an apparatus wherein, as a solution to this problem, the receptacle is provided with a cover with an opening for escape of the steam through a water-spray chamber which removes the contaminants. This leaves the problem of disposing of the contaminated water, adds complexity and cost, and adds still further to maintenance problems.